1. Field of the Invention
The present invention relates to low alloy steel used as a material for steam turbines or the like, and more specifically to nickel-chrome-molybdenum steel.
2. Description of the Prior Art
Generally, for materials used for steam turbines driven by water vapor having a high temperature and a high pressure (approximately 300.degree. C. and 70 kg/cm.sup.2), excellent strength and toughness over a wide range of temperatures are required, and nickel-chrome-molybdenum steel to which vanadium is added, which is high strength steel, is used as a material to meet th aforesaid characteristic. This steel is obtained by adding molybdenum or vanadium which is a fine carbide deposited element to nickel-chrome high strength steel sensitive to temper embrittlement as is known whereby increasing a restraint of softening, that is, a tempering resistance at a high tempering temperature. This steel is well suited for the above-described applications.
It has been however recently revealed that stress corrosion cracking are frequently encountered in low pressure steam turbines and peripheral equipment which are nickel-chrome-molybdenum steel with vanadium added thereto mainly in the United States and European nuclear power stations, which poses a significant problem. This stress corrosion cracking occurs mainly in a key way at which a disk and a shaft are secured together and in a joint between a blade and a disk. It is said that Na in the form of impurities in vapor is concentrated as NaOH in crevices of such portions as described above to form a crack along a grain boundary along with the presence of a high load stress when the turbine is operated. It has been also known that intergranular stress corrosion cracking occurred in carbon steel subjected to stress and to the environment containing OH. In view of the foregoing, it has been earnestly desired to develop nickel-chrome-molybdenum steel having excellent stress corrosion cracking resistance even under the severe using environment.